Method for inhibiting abnormal expansion of super-high-grade pellets through gradient heating reduction of hydrogen-based shaft furnace

Abstract

The invention discloses a method for inhibiting abnormal expansion of ultrahigh-grade pellets through gradient heating reduction of a hydrogen-based shaft furnace, and belongs to the technical field of direct reduction iron production. The method comprises the following steps: continuously adding ultrahigh-grade pellets from the top of a hydrogen-based shaft furnace, and sequentially passing through a first heating reduction zone (the heating speed is 10-20 DEG C / min, and the temperature reaches 900-930 DEG C), a second heating reduction zone (the heating speed is not less than 30 DEG C / min, and the temperature reaches 990-1020 DEG C), a constant-temperature reduction zone and a cooling zone of the hydrogen-based shaft furnace to obtain the direct reduction iron. The method is suitable for the hydrogen-based shaft furnace with various reducing atmospheres, the maximum reduction expansion rate of the ultrahigh-grade pellets in the reduction process of the hydrogen-based shaft furnace can be smaller than 15% on the premise that the original iron grade of the pellets is not reduced, high-purity direct reduction iron with qualified production indexes is produced, and reduction of follow-up electric furnace smelting energy consumption and the molten iron refining process are facilitated.

Description

Technical Field

[0001] The invention relates to a method for hydrogen-based shaft furnace reduction of ultra-high-grade pellets, and in particular to a method for gradient temperature-raising reduction in a hydrogen-based shaft furnace to suppress abnormal expansion of ultra-high-grade pellets, and belongs to the technical field of direct reduced iron production. Background Art

[0002] The hydrogen-based vertical furnace-electric furnace short-process smelting process is the core path for the steel industry to achieve green and low-carbon transformation. With the increasing reduction of high-quality lump ore resources, pellets have gradually become the main charge of the hydrogen-based vertical furnace process. Due to the reduction characteristics of the hydrogen-based vertical furnace, the gangue components in the pellets cannot be reduced or removed by slagging, so that the direct reduced iron will retain the original impurity elements and harmful metal elements in the pellets, thus entering the subsequent electric furnace process, increasing the smelting energy consumption and process. The use of ultra-high-grade pellets (gangue content less than 0.5%) to prepare direct reduced iron can improve the purity of molten iron, reduce the amount of electric furnace slag, and reduce smelting energy consumption and costs, thereby laying the foundation for realizing low-carbon or carbon-free electric furnace steelmaking and producing high-quality clean steel and special steel. However, there is a technical bottleneck of abnormal expansion of ultra-high-grade pellets in the hydrogen-based vertical furnace reduction process, which is difficult to meet production requirements. The literature (“Experimental study on a new process for preparing high-purity iron by direct reduction of ultra-high-grade iron concentrate-melting”, Zhao Jiaqi, Northeastern University, Master’s thesis, 2017) disclosed that the pellet expansion rate was greater than 50% when ultra-high-grade pellets were subjected to hydrogen reduction, and the pellet strength after reduction was less than 200 N. The literature (“Mechanism study on gas-based reductionswelling behavior of ultra-high grade pellets”, Jie Lei, et al., Journal of Materials Research and Technology; 2023; 26:823-836) disclosed that the pellet reduction expansion rate was greater than 30% when high-grade pellets were subjected to hydrogen reduction. Although the use of calcium and magnesium additives can inhibit the reduction expansion of ultra-high-grade pellets to a certain extent, it will inevitably reduce the iron grade of the pellets, affect the reduction rate of the pellets, and increase the hydrogen consumption during the reduction process. Summary of the invention

[0003] Aiming at the problem of abnormal expansion of ultra-high-grade pellets (gangue content less than 0.5%) in the hydrogen-based vertical furnace reduction process in the prior art. The purpose of the present invention is to provide a method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace. The key to this method is to regulate the reduction process of ultra-high-grade pellets, perform gradient temperature reduction in a hydrogen-based vertical furnace, and suppress abnormal expansion of pellets caused by crystal transformation and iron whisker growth while ensuring the reduction efficiency of pellets. This method is particularly suitable for hydrogen-based vertical furnaces with a variety of reducing atmospheres. Without reducing the original iron grade of the pellets, the maximum reduction expansion rate of ultra-high-grade pellets in the hydrogen-based vertical furnace reduction process can be less than 15%, and high-purity direct reduced iron with qualified production indicators can be produced, which is conducive to reducing the subsequent electric furnace smelting energy consumption and molten iron refining process.

[0004] The ultra-high-grade pellets involved in the present invention are a common definition in the industry, referring to pellets with a gangue content of less than 0.5%.

[0005] In order to achieve the above technical purpose, the present invention provides a method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace, wherein the ultra-high-grade pellets are continuously added from the top of the hydrogen-based vertical furnace, and sequentially pass through the first temperature-raising reduction zone, the second temperature-raising reduction zone, the constant temperature reduction zone and the cooling zone of the hydrogen-based vertical furnace to obtain direct reduced iron;

[0006] In the first temperature-raising reduction zone, the ultra-high-grade pellets are heated at a rate of 10-20°C / min. When the ultra-high-grade pellets are heated to 900-930°C and the reduction degree of the ultra-high-grade pellets reaches more than 40%, they enter the second temperature-raising reduction zone;

[0007] In the second temperature-raising reduction zone, the heating rate of the ultra-high-grade pellets is not less than 30°C / min. When the ultra-high-grade pellets are heated to 990-1020°C, they enter the constant temperature reduction zone.

[0008] The key improvement of the technical solution of the present invention is that a gradient temperature rise reduction method is adopted in the hydrogen-based vertical furnace reduction process, with the purpose of matching the temperature field of the pellets with the reduction process of the pellets during the hydrogen-based vertical furnace reduction process. On the one hand, the expansion of the pellets caused by the internal stress of the crystal transformation in the early stage of reduction is reduced. On the other hand, the initial morphology of the iron grains on the surface of the particles in the pellets is regulated to induce uniform nucleation and aggregation growth of the iron grains, and to inhibit the abnormal expansion of the pellets caused by the growth of iron whiskers. More specifically, the present invention can reduce the expansion stress of the pellets while ensuring high reduction efficiency by regulating the appropriate gradient temperature rise rate of the pellets with different reduction processes. First, a relatively low temperature rise rate is controlled by the first temperature rise reduction zone, the reduction time of the pellets is extended, and the reduction degree of the pellets after reduction is controlled to be not less than 40%. On the one hand, Fe can be reduced. 2 O3 Reduction to Fe 3 O 4 On the other hand, it can induce uniform nucleation of iron grains on the surface of particles in the pellets, and promote the aggregation and growth of iron grains in the pellets during the subsequent isothermal reduction process; on the other hand, it can form an initial metal iron skeleton structure before the pellets enter the isothermal reduction, and resist the structural stress and expansion caused by the growth of iron grains during the high-temperature reduction process. Secondly, based on the improvement of the reduction degree of the pellets after reduction in the first temperature-raising reduction zone, it is beneficial to inhibit the abnormal expansion of the pellets, but it will inevitably increase the reduction time of the pellets and affect the subsequent reduction efficiency. Therefore, the use of a faster heating rate in the second temperature-raising reduction zone can enable the pellets to avoid the temperature range where abnormal expansion is prone to occur. Thirdly, heating the pellets to 990~1020℃ and then entering the isothermal reduction zone for continued reduction is not only conducive to accelerating the reduction of fusite to precipitate metallic iron, but also the higher temperature is conducive to the aggregation and growth of iron grains, thereby forming a tighter pellet structure and realizing the gradual steady-state reduction of the pellets. In summary, the present invention uses a gradient temperature reduction method to reduce the maximum reduction expansion rate of the pellets in the hydrogen-based shaft furnace reduction process to less than 15% without affecting the original iron grade and reduction performance of the pellets.

[0009] As a preferred solution, in the first temperature-raising reduction zone, when the reduction degree of the ultra-high-grade pellet reaches 40-70%, it enters the second temperature-raising reduction zone. If the ultra-high-grade pellet has a low reduction degree in the first temperature-raising reduction zone, it is difficult to induce uniform nucleation of iron grains on the surface of particles in the pellet, which is not conducive to the aggregation and growth of iron grains in the pellet in the subsequent isothermal reduction process, and it is impossible to form a preliminary metallic iron skeleton structure, and it is difficult to resist the structural stress and expansion caused by the growth of iron grains in the high-temperature reduction process. But when the ultra-high-grade pellet has a high reduction degree in the first temperature-raising reduction zone, on the one hand, it will significantly increase the reduction time of the pellet and reduce the yield, and on the other hand, it is not conducive to the aggregation and growth of iron grains in the ultra-high-grade pellet.

[0010] As a preferred solution, in the second temperature-raising reduction zone, the heating rate of the ultra-high-grade pellets is 30-50°C / min. If the heating rate is too low or too high, the pellets will expand abnormally.

[0011] As a preferred solution, the ultra-high grade pellets (SiO 2 +CaO+MgO+Al 2 O 3 ) is less than 0.5%, the mass fraction of (P+S) is less than 0.005%, and the mass fraction of TFe is greater than 69.5%.

[0012] As a preferred solution, the compressive strength of the ultra-high grade pellets is 2500-3000 N, and the reduction index RI is 40 Greater than 3. The strength of the ultra-high-grade pellets of the present invention should not be too high. When the strength is too high, the pellet structure is too dense, and the pore structure of the inner and outer layers of the pellets is uneven. On the one hand, the reduction rate of the pellets will be reduced. On the other hand, the structural stress caused by the difference in the inner and outer structures of the pellets during the reduction process will be induced, thereby causing expansion and cracking. Controlling the strength of the ultra-high-grade pellets within the range of 2500~3000N while having good reduction performance can ensure its reduction efficiency during the gradient temperature reduction process and reduce the reduction expansion of the pellets. The reduction index RI 40 It can be measured according to GB-T 24236-2009.

[0013] As a preferred solution, the ultra-high-grade pellets are preheated to 550-650° C. before entering the hydrogen-based shaft furnace. Properly increasing the temperature of the pellets entering the furnace can reduce the carbon deposition reaction of the hydrogen-rich gas, improve the utilization rate of the reducing gas, and effectively inhibit the expansion of the pellets caused by the carbon deposition reaction on the crystal transformation.

[0014] As a preferred solution, the reducing gas introduced into the first temperature-raising reduction zone, the second temperature-raising reduction zone and the constant temperature reduction zone is composed of: (H 2 +CO) accounts for no less than 90% by volume, H 2 / (H 2 +CO) volume ratio is not less than 0.6. The present invention can be applied to the hydrogen shaft furnace reduction process of various reducing gas components. Sufficient reducing gas content can accelerate the reduction rate of ultra-high-grade pellets and regulate the growth morphology of granular iron grains. 2 In addition to the two main gases, CO and CO, other gases may be inert gases or nitrogen, etc.

[0015] As a preferred solution, the reducing gas flow rate is 1600-2000m3 per ton of direct reduced iron. 3 According to the measurement, the gas pressure in the hydrogen-based vertical furnace is 0.5~0.9 MPa. The appropriate reducing gas flow rate and gas pressure in the furnace can ensure sufficient reduction potential and improve gas utilization.

[0016] The ultra-high-grade pellets reduced in the constant temperature reduction zone of the present invention have a reduction degree greater than 98%.

[0017] As a preferred solution, the TFe of the direct reduced iron is greater than 99%, and the compressive strength is greater than 400 N. The high-purity direct reduced iron prepared by the present invention has the advantages of low impurity content, compact structure, good strength, etc., and its TFe is greater than 99%, and the compressive strength is greater than 400 N. It can be used as a high-quality furnace material for low-carbon steelmaking and carbon-free steelmaking, which is beneficial to reducing smelting energy consumption and simplifying the molten iron refining process.

[0018] Compared with the prior art, the beneficial technical effects brought by the technical solution of the present invention are:

[0019] The present invention establishes a matching relationship between the temperature gradient of the hydrogen-based vertical furnace and the reduction process of the ultra-high-grade pellets, regulates the appropriate gradient heating rate and reduction degree of the pellets in different reduction processes, and suppresses the abnormal expansion of the ultra-high-grade pellets under the premise of ensuring the reduction efficiency. The method does not need to add additives such as calcium and magnesium to the ultra-high-grade pellets. Under the premise of not reducing the original iron grade of the ultra-high-grade pellets, the reduction expansion rate of the pellets in the hydrogen-based vertical furnace reduction process can be less than 15%, solving the problem of abnormal expansion in the hydrogen-rich reduction process of the ultra-high-grade pellets. In addition, by setting a suitable gradient heating interval, the aggregation and growth of iron grains are promoted to form a tighter pellet structure, and high-purity direct reduced iron with qualified quality indicators is produced, which is conducive to reducing the subsequent electric furnace production energy consumption and molten iron refining process. DETAILED DESCRIPTION

[0020] In order to further illustrate the content of the present invention, the present invention will be described more comprehensively and meticulously in combination with preferred embodiments below, but the protection scope of the present invention is not limited to the following specific embodiments.

[0021] Unless otherwise defined, all the professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0022] Table 1 shows the main chemical composition of the ultra-high grade pellets used.

[0023]

[0024] Example 1

[0025] The method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace provided in this embodiment uses ultra-high-grade pellets with a compressive strength of about 2800N and a reduction index RI of about 1.38. 40 The reducing gas composition in the first temperature-increasing reduction zone, the second temperature-increasing reduction zone and the constant temperature reduction zone during the gradient temperature-increasing reduction process is: (H 2 +CO) accounts for 90% by volume, H 2 / (H2 +CO) volume ratio is 1 (100%H 2 The reducing gas flow rate is 1600 m3 / t DRI. 3 , the gas pressure in the furnace is 0.9MPa.

[0026] First, the ultra-high-grade pellets are preheated to 550°C outside the furnace, and then enter the first temperature-raising reduction zone and reduce to 930°C at a rate of 20°C / min. At this time, the reduction degree of the pellets is 41.6%, and then enter the second temperature-raising reduction zone; in the second temperature-raising reduction zone, the temperature is increased to 990°C at a rate of 30°C / min, and then enter the constant temperature reduction zone; the pellets are reduced at a constant temperature at 990°C until the reduction degree of the pellets is 99.2%. The maximum reduction expansion rate of the pellets during the reduction process is 13.2%, and the obtained high-purity direct reduced iron TFe is 99.32%, and the compressive strength is 417 N.

[0027] Example 2

[0028] The method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace provided in this embodiment uses ultra-high-grade pellets with a compressive strength of about 2800N and a reduction index RI of about 1.38. 40 The reducing gas composition of the first temperature-increasing reduction zone, the second temperature-increasing reduction zone and the constant temperature reduction zone during the gradient temperature-increasing reduction process is: (H 2 +CO) accounts for 95% by volume, H 2 / (H 2 +CO) volume ratio is 0.6. The reducing gas flow rate is 2000 m per ton of direct reduced iron. 3 , the gas pressure in the furnace is 0.5 MPa.

[0029] First, the ultra-high-grade pellets are preheated to 650℃, then enter the first temperature-raising reduction zone and reduce to 900℃ at a rate of 10℃ / min, at which time the reduction degree of the pellets is 43.8%, and then enter the second temperature-raising reduction zone; in the second temperature-raising reduction zone, the temperature is increased to 1020℃ at a rate of 50℃ / min, and then enter the constant temperature reduction zone; the pellets are reduced at a constant temperature at 1020℃ until the reduction degree of the pellets is 98.3%. During the reduction process, the maximum reduction expansion rate of the pellets is 13.8%, and the obtained high-purity direct reduced iron TFe is 99.12% and the compressive strength is 405N.

[0030] Example 3

[0031] The method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace provided in this embodiment uses ultra-high-grade pellets with a compressive strength of about 2800 N and a reduction index RI of 40The reducing gas composition in the first temperature-increasing reduction zone, the second temperature-increasing reduction zone and the constant temperature reduction zone during the gradient temperature-increasing reduction process is: (H 2 +CO) accounts for 95% by volume, H 2 / (H 2 +CO) volume ratio is 0.72. The reducing gas flow rate is 1800m3 / ton of direct reduced iron. 3 , the gas pressure in the furnace is 0.7MPa.

[0032] First, the ultra-high-grade pellets are preheated to 600℃, then enter the first temperature-raising reduction zone and reduce to 900℃ at a temperature of 15℃ / min, at which the reduction degree of the pellets is 57.2%, and then enter the second temperature-raising reduction zone; in the second temperature-raising reduction zone, the temperature is increased to 1020℃ at a temperature of 40℃ / min, and then enter the constant temperature reduction zone; the pellets are reduced at a constant temperature of 1020℃ until the reduction degree of the pellets is 99.8%. The maximum reduction expansion rate of the pellets during the reduction process is 8.7%, and the obtained high-purity direct reduced iron TFe is 99.46% and the compressive strength is 443N.

[0033] Example 4

[0034] The method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace provided in this embodiment uses ultra-high-grade pellets with a compressive strength of about 2700N and a reduction index RI of about 1.38. 40 The reducing gas composition in the first temperature-increasing reduction zone, the second temperature-increasing reduction zone and the constant temperature reduction zone during the gradient temperature-increasing reduction process is: (H 2 +CO) accounts for 95% by volume, H 2 / (H 2 +CO) volume ratio is 0.86. The reducing gas flow rate is 1900m3 / ton of direct reduced iron. 3 , the gas pressure in the furnace is 0.7MPa.

[0035] First, the ultra-high-grade pellets are preheated to 550°C, then enter the first temperature-raising reduction zone and reduce to 930°C at a rate of 10°C / min, at which point the reduction degree of the pellets is 68.2%, and then enter the second temperature-raising reduction zone; in the second temperature-raising reduction zone, the temperature is raised to 1020°C at a rate of 40°C / min, and then enter the constant temperature reduction zone; the pellets are reduced at a constant temperature of 1020°C until the reduction degree of the pellets is 98.6%. During the reduction process, the maximum reduction expansion rate of the pellets is 7.4%, and the obtained high-purity direct reduced iron TFe is 99.15%, and the compressive strength is 418N.

[0036] Comparative Example 1

[0037] Compared with Example 1, the only difference is that the ultra-high-grade pellets enter the first temperature-raising reduction zone and are reduced at a temperature of 30°C / min to 930°C, at which time the reduction degree of the pellets is 32.2%, and then enter the second temperature-raising reduction zone. During the reduction process, the maximum reduction expansion rate of the ultra-high-grade pellets is 25.3%, and the compressive strength of the obtained high-purity direct reduced iron is 298N.

[0038] Compared with Example 1, due to the faster heating rate used in the first temperature-raising reduction zone, the nucleation of iron grains on the surface of the particles is uneven in the initial stage of fusite reduction, which induces the formation of iron whiskers during the isothermal reduction process, increases the stress in the pellets, causes abnormal expansion and cracking, and reduces the compressive strength.

[0039] Comparative Example 2

[0040] Compared with Example 1, the only difference is that the pellets are heated to 950° C. in the II temperature-raising reduction zone and then enter the constant temperature reduction zone, so that the pellets are reduced at a constant temperature of 950° C. During the reduction process, the maximum reduction expansion rate of the ultra-high-grade pellets is 21.6%, and the compressive strength of the obtained high-purity direct reduced iron is 366N.

[0041] Compared with Example 1, the pellets after gradient temperature increase are reduced at a constant temperature of 950° C. This temperature is conducive to the growth of iron whiskers on the particle surface, inhibits the aggregation and growth of iron grains, and causes abnormal expansion of the pellets.

[0042] Comparative Example 3

[0043] Compared with Example 1, the only difference is that the ultra-high grade pellets used have a compressive strength of about 3800N and a pellet reduction index of RI 40 The maximum reduction expansion rate of the ultra-high-grade pellets during the reduction process is 16.7%, and the compressive strength of the obtained high-purity direct reduced iron is 385N.

[0044] Compared with Example 1, the ultra-high-grade pellets with higher compressive strength were used, and the reduction performance of the pellets was reduced due to the denser pore structure of the pellets. Due to the difference in pore structure between the inner and outer layers of the pellets, the structural stress inside the pellets increased during the reduction process, causing the pellets to expand and crack.

[0045] Comparative Example 4

[0046] Compared with Example 2, the only difference is that the ultra-high grade pellets are preheated to 450°C before entering the first temperature-increasing reduction zone. During the reduction process, the maximum reduction expansion rate of the ultra-high grade pellets is 22.5%, and the compressive strength of the obtained high-purity direct reduced iron is 326N.

[0047] Compared with Example 2, when the pellet entering the furnace temperature is 450°C, an obvious carbon precipitation reaction occurs in the pellets at the initial stage of reduction, which increases the expansion of the pellets caused by the crystal transformation.

[0048] Comparative Example 5

[0049] Compared with Example 1, the only difference is that the reducing gas composition during the gradient temperature reduction process is: H 2 / (H 2 +CO) volume ratio is 0.5. The maximum reduction expansion rate of the ultra-high-grade pellets during the reduction process is 25.8%, and the compressive strength of the obtained high-purity direct reduced iron is 323N.

[0050] Compared with Example 1, when the CO ratio in the reducing gas is too high, the reduction rate of the pellets is reduced, and an obvious carbon precipitation reaction occurs in the pellets at the initial stage of reduction, which increases the expansion of the pellets caused by the crystal transformation.

[0051] Comparative Example 6

[0052] Compared with Example 1, the only difference is that the reducing gas flow rate during the gradient temperature reduction process is 1400m3 / t of direct reduced iron. 3 The maximum reduction expansion rate of the ultra-high-grade pellets during the reduction process was 22.3%, and the compressive strength of the obtained high-purity direct reduced iron was 355N.

[0053] Compared with Example 1, the reduction rate of the pellets decreased due to the reduction ratio of reducing gas, which was not conducive to the uniform nucleation of iron grains on the particle surface during the gradient temperature reduction process, causing the formation of iron whiskers and resulting in abnormal expansion of the pellets.

Claims

1. A method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based shaft furnace, characterized in that: The ultra-high-grade pellets are continuously added from the top of the hydrogen-based vertical furnace, and sequentially pass through the first temperature-raising reduction zone, the second temperature-raising reduction zone, the constant temperature reduction zone and the cooling zone of the hydrogen-based vertical furnace to obtain direct reduced iron; In the first temperature-raising reduction zone, the ultra-high-grade pellets are heated at a rate of 10-20°C / min. When the ultra-high-grade pellets are heated to 900-930°C and the reduction degree of the ultra-high-grade pellets reaches more than 40%, they enter the second temperature-raising reduction zone; In the second temperature-raising reduction zone, the heating rate of the ultra-high-grade pellets is not less than 30°C / min. When the ultra-high-grade pellets are heated to 990-1020°C, they enter the constant temperature reduction zone.

2. The method of suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace according to claim 1, characterized in that: The mass fraction of (SiO2+CaO+MgO+Al2O3) in the ultra-high-grade pellet is less than 0.5%, the mass fraction of (P+S) is less than 0.005%, and the mass fraction of TFe is greater than 69.5%.

3. A method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace according to claim 1 or 2, characterized in that: The compressive strength of the ultra-high grade pellets is 2500~3000 N, and the reduction index RI is 40 Greater than 3.

4. A method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace according to claim 1 or 2, characterized in that: The ultra-high-grade pellets are preheated to 550-650° C. before entering the hydrogen-based vertical furnace.

5. A method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace according to claim 1 or 2, characterized in that: The reducing gas introduced into the I temperature rising reduction zone, the II temperature rising reduction zone and the constant temperature reduction zone has the following composition: the volume ratio of (H2+CO) is not less than 90%, and the volume ratio of H2 / (H2+CO) is not less than 0.

6.

6. The method of suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace according to claim 5, characterized in that: The flow rate of the reducing gas is 1600-2000 m3 per ton of direct reduced iron. 3 According to measurement, the gas pressure in the hydrogen-based vertical furnace is 0.5~0.9 MPa.

7. A method for suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature reduction in a hydrogen-based vertical furnace according to claim 1, 2 or 6, characterized in that: The TFe of the direct reduced iron is greater than 99%, and the compressive strength is greater than 400 N.

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